This invention relates to optical scanning systems utilizing a beam of collimated radiant energy and, more particularly, to a beam position detection system for use therein.
In recent years, optical scanning systems have become widely used, such as in copiers, duplicators, facsimile transmission devices and laser recording and projecting systems, an example of the latter being disclosed in U.S. Pat. No. 4,345,258, which issued to R. C. Tsai and W. J. Kidwell on Aug. 17, 1982. In these scanning systems, a collimated light beam, typically a laser generated beam, is scanned, or deflected, across a surface by means of a galvanometer-mounted mirror. In such a system, it is often necessary to be able to determine the instantaneous position of the beam as it scans the surface. It is therefore a primary object of this invention to provide a beam position detection system for use in an optical scanning system.
One prior approach was to use a pair of beams which are scanned across a grating in synchronism with the main beam. The two beams are focused at the plane of the grating and are separated, at this plane, by a distance equal to 1/4 of the grating period. When the main beam is moving in a first direction, one of the two beams will be blocked by the grating first and when the main beam is moving in the opposite direction, the other of the two beams will be blocked by the grating first. The two beams are detected by separate detection elements, for example at different frequencies, and the outputs of the detection elements are interpreted to determine the direction of motion as well as generating counting pulses to provide an incremental distance. While effective, such an approach is difficult and expensive to implement, it requiring two precisely positioned laser beams. Additionally, such a system is susceptible to noise due to deterioration of the grating. It is therefore a further object of this invention to provide a beam position detection system which is relatively inexpensive and simple to implement.